The purpose of this pilot project is to determine the influence of age on the ability to adjust nutrient oxidation to acute changes in diet composition. Our hypotheses are that older humans: 1. Will adjust nutrient oxidation in response to a mixed diet condition similarly; 2. Will demonstrate a less rapid increase in fat oxidation and a more positive fat balance in response to a high fat diet condition; and 3. Will demonstrate a more rapid increase in carbohydrate condition compared to the young subjects. Subjects ar undergoing four whole room calorimetry visits. The first calorimeter visit is being used to carefully determine individual energy requirements which will be used for subsequent calorimeter visits. Each of the three subsequent calorimeter visits consists of a mixed diet, high carbohydrate, and a high fat diet which are being administered in a randomized design. Blood samples are being drawn to measure changes in circulating hormone and substrate levels after each calorimeter visit. Skeletal muscle and adipose tissue biopsies are also being taken after each diet condition. Adipose tissue and skeletal muscle lipoprotein lipase and skeletal muscle citrate synthase, phosphofructokinase, and b- khydroxyacetyl co a dehydrogenase activities will be measured to determine if these markers for adipose tissue and skeletal muscle metabolism are an important source of between subject variability in the whole body nutrient oxidation adjustments.